Cheaper store purchases
Get Yuzu as a partner by finding her in Takatane Village while you have less than 300 HP. Talk to her and select the "Oooh....Mother." option. After she heals you, talk to her again to have her join. While she is your partner, you can repeatedly talk to a vendor to get a random discount.

Additionally there are three secret trophies
Samurai Slayer (Bronze): Complete the game in Instant Kill difficulty.
Victory (Gold): Completed and discovered the True Ending.
Samurai 4EvR! (Gold): Cleared game with more than 120% Samurai Rating.